For a better understanding of the background from which the invention springs, but with no prejudice to the generalities expressed in the claims accompanying the present specification, the introduction which follows will relate to time-resolved FT-IR spectroscopy, which is becoming widely practised and shows a promising future.
In a non-time-resolved regular FT-IR spectrometer, introduced here by way of background information, an interferometer of the Michelson type splits a polychromatic input beam into a reflected beam and a transmitted beam by means of a beam splitter. Each split beam travels along its own path to a return mirror which deflects it back to the beam splitter along the same path. One of the return mirrors is stationary, whilst the other is movable along a rectilinear track between two mirror travel limits equidistant from a datum position therebetween. At the beam splitter, the two returned split beams recombine along a co,non output path leading to a photodetector via a sample station.
If the movable mirror is adjusted so that the optical path length from beam splitter to return mirror and from the latter back to the beam splitter is exactly the same for the two beams, i.e. if the movable mirror is located at its datum position, then the two halves of each constituent split optical wave, one half in one split beam and the other half in the other split beam, will undergo constructive interference, which means that respective wavefronts will overlap. In other words, at the datum position or, more specifically, zero OPD (Optical Path Difference) position, all the constituent waves of the input beam which were split by the beam splitter will recombine simultaneously, as shown by the dominant signal produced by the photodetector. This intense signal is referred to in the art as the centreburst.
If the movable mirror is now shifted towards the incoming split beam, the optical path length "seen" by the movable mirror is decreased; conversely, it will be increased if the mirror is moved in the opposite direction. A mirror travel from one to other limit will therefore generate two complete series of OPD values of opposite signs, as required for Fourier transformation, presently to be introduced. Such travel is referred to as an OPD scan. Each OPD change from the zero OPD position of the movable mirror corresponding to one half wavelength of a constituent optical wave will produce a sinusoidal optical modulation of the wave varying at recombination from a maximum when the two split waves are in phase (constructive interference) to a minimum when they are in phase opposition (destructive interference). In terms of the photodetector signal, it means that as the OPD scan proceeds a series of superimposed electrical sine waves will be generated of different frequencies (known as Fourier frequencies) and amplitudes. That signal represents an interferogram.
So far no reference has been made to the presence of a sample at the sample station. If a sample is inserted, the interferogram taken is that of the sample superimposed on that of the source. If no sample is inserted, the resulting interferogram is that of the source, of course. By taking the Fourier Transform of the former interferogram and, separately, that of the latter and ratioing the two transforms, the spectrum of the sample is obtained.
It is desirable to emphasize that an interferogram is sampled as a series of elemental data (hereinafter data points) and the OPD scan from mid-scan to either scan limit is divided into a corresponding series of equal OPD increments (hereinafter OPD points), each in coincidence with a zero crossing of a reference laser interferogram. A data point occurring at a given position in the series of data points is always sampled at the OPD point occupying the same position in the series of OPD points. Thus data point 1 is always sampled at OPD point 1, data point 2 at OPD point 2, and so on.
Fuller background details are contained in the introduction to U.S. Pat. No. 4,684,255, which is imported in full into this specification and is hereinafter referred to as Imported Patent.
Time-resolved FT-IR spectrophotometers are known in which to the multiple modulation covering a band of Fourier frequencies provided by the interferometer (hereinafter referred to as interferometer modulation) in translating each optical wave present in the interferometer input beam into an electrical sine wave, the amplitude and frequency of which are related to amplitude and frequency of the optical wave, there is added a cyclic stretching and relaxing of the sample in the shape of a thin strip by means of a rheometer. These mechanical cyclic perturbations are hereinafter referred to as sample modulation. Depending on the nature of the sample, the perturbations may cause changes in certain constituent dipole moments of a molecule and, consequently, in the dipole moment of the molecule as a whole.
Whenever the dipole moment of a molecule changes, absorption takes place. The change in dipole moment brought about by sample modulation may be regarded as "dynamic" absorption to distinguish it from the "static" absorption of conventional FT-IR spectroscopy, wherein an unperturbed sample is used. By accumulating data at each successive OPD points of an OPD scan by the interferometer which synchronize with sample strain resulting from the stress applied by the cyclic perturbations, it is possible to derive a spectrum on the time-dependence of absorbance, which when compared with the static spectrum provides information useful in the interpretation of the latter e.g. in resolving a featureless absorbance band into a number of constituent peaks.
Another well known FT-IR technique is concerned with Infra Red Linear Dichroism (IRLD). Samples that absorb light differentially between two orthogonal components of linearly polarized light are said to exhibit dichroism. This effect occurs naturally in certain crystalline materials, such as tourmaline, and may be induced by stretching in others, such as atactic polystyrene. In this specification, the phrase dichroic sample shall be understood to refer to a sample in which dichroism is either natural or induced.
In regular IRLD, an ultrasonic photo-elastic modulator, referred to as the PEM, causes an interferometer output beam that has been linearly polarized to alternate between two orthogonal linearly polarized states at an ultrasonic frequency. The sample is so orientated that its dichroic axis is either parallel or perpendicular to the modulation axis of the PEM. As it passes through the sample, the polarization modulated beam is differentially absorbed between the two orthogonal polarization states. A detector receiving the emerging beam yields an electrical output in which a wave at the ultrasonic polarization frequency appears atop a waveform representing the emission interferogram of the interferometer source compounded with the regular IR absorbance interferogram of the sample. The linear dichroism difference information is contained in the modulation of the ultrasonic wave. The "envelope" of this modulation may be extracted and processed by known means to provide the linear dichroism difference spectrum of the sample.
Another FT-IR technique results from the combination of polarization modulation with sample modulation. It is known as DIRLD, which stands for Dynamic Infra-Red Linear Dichroism, in contradistinction to IRLD which may be thought of as "static" IRLD. Now the modulation of the ultrasonic wave in the detector output contains information on dichroism components respectively in phase and in quadrature with sample strain. This information may be extracted by known methods and displayed as dynamic dichroic difference spectra side by side with the static dichroic difference spectrum of the sample.
In the application of the prior art FT-DIRLD technique the aim has been to establish the time dependence of absorbance in a dichroic sample. It was soon realized that if the interferometer band of modulation frequencies was too closely spaced from the sample modulation frequency, sidebands of the former would interfere with certain frequencies within the said band. Unfortunately, the obvious remedy of distancing the sample modulation frequency sufficiently to avoid interference effects was not available because the choice of such frequency is governed by the requirements of the analysis to be undertaken and the nature of the sample. The answer was to replace continuous OPD scanning by step scanning. It has meant: stopping the scan at the first OPD point of a complete scan for the duration of one cycle or more of the sample modulation; sampling the interferogram for various phase angles of the or each sample modulation cycle; repeating the process at each subsequent OPD point until completion of the scan; and analyzing the data.
Unfortunately, the step scanning system required hitherto is complex and therefore expensive; more importantly, OPD scanning can take several hours and no spectra is produced till the very end. The prior art solution avoids the interference problem referred to earlier but incurs a severe penalty. Not being able to use the fast scan facilities is also a serious drawback in time resolved FT spectroscopy.